An optical disk or compact disk (CD) is a nonmagnetic data storage medium on which relatively large amounts of digital information can be stored by using a laser beam to burn microscopic indentions into the surface of the medium. The stored data can be read using a lower-power laser to sense the presence or absence of the indentions. A compact disk read-only-memory (CD-ROM) is a play only optical storage medium onto which data can be stored only once and then read repetitively. A compact disk direct-read-after-write (CD-DRAW), also referred to as a write-once read-many (WORM) disk, is a similar optical storage medium especially adapted so that data can be written onto the medium by a user. Also available are opto-magnetic disks on which both data recording and data reproduction are feasible by the user.
All of the above optical storage media offer improved density storage capacity over conventional magnetic tape or disk media. For example, a five-inch CD-ROM disk is capable of storing as much as 620 megabytes of data while a standard, 51/4 inch magnetic floppy disk can only store 1.2 megabytes of data. Such storage capacity, available in a relatively small removable medium, has led to a wide variety of applications for the optical storage and subsequent processing of data. Compact disks and associated processors embodying this technology are available for applications that include audio and video recording and playback, computer animation, interactive and multimedia computer games, text storage and other computer program, data and graphics applications. A commercially available audio CD, for example, is a 5-inch disk capable of storing over 72 minutes of continuous music for reproduction on a playback device.
Due to the popularity of the audio CD, an industry standard has been developed for compact disk specifications, recording format and error detection/correction techniques, driven primarily by the requirements for storing and reproducing audio information. For instance, data on a CD is stored in one long continuous track extending spirally from the center of the disk, divided along the circumferential direction thereof into a plurality of equiangular unit storage regions or sectors. Data is stored in this manner to allow an optical read head of a CD reader drive to position itself at a location on the track, and then follow that track from beginning to end, delivering continuous music without breaks or gaps in the sound. While such a format is ideal for storing data that is contiguous in nature, it creates problems when the stored data requires intermittent processing.
In general purpose computer systems, the requirements for accessing data are different from that of the audio market. Rarely will data that is placed on a floppy or hard disk be read in a contiguous fashion. Rather, the computer will request a portion of data, process that data, request more data, process that and so on. Thus, to allow for such intermittent reading and processing, floppy and hard disks are designed to provide for quick access to the data stored on them.
To allow for such quick access, data is stored on floppy and hard disks in a plurality of concentric tracks with the tracks divided into a plurality of equally spaced sectors. Since the tracks on floppy and hard disks are concentric, the exact radial position of each track can be easily calculated. Thus, when a computer needs to read particular data from a floppy or hard disk, the location of the track containing the data is calculated, the read head of the disk drive is positioned over the track and the data is provided to the computer.
Unlike floppy or hard disks, the exact position of a particular track or sector on a CD cannot be easily calculated since the radial position of the track varies with the rotation of the disk. As a result, when a computer requests information from a CD, the optical read head cannot be placed directly over the track containing the required data. Instead, the radial position of the track containing the data is approximated the optical read head is placed one or two spirals in front of the track containing the data, and the read head follows the track until the desired data is reached. Such approximation in locating the track containing the data stored on a CD leads to delays or gaps in processing the desired data, as well as gaps in the presentation of processed data to the user, all of which are unacceptable in many applications.
The foregoing problems are likely to be manifested in a system for reproducing both video and audio information contained on a CD, such as in a multimedia computer system. A CD for a multimedia computer system containing data for a video presentation, game or the like contains both audio and video data interleaved throughout the data sectors on the disk (e.g., audio sector, video sector, video sector, audio sector, etc.). When the optical read head of a CD drive is positioned at the beginning of a track, it delivers data to a multimedia computer that is responsible for both playing audio and displaying video. Due to the processing requirements for displaying video, the multimedia computer may not be able to process the data received from the CD drive as fast as it is read by the drive. Thus when the multimedia computer has completed processing a portion of the video and is ready for the next sector of data, the CD drive will have passed the sector containing the desired data. The multimedia computer must therefore initiate a "reseek" of the data, requiring the CD drive to reposition the optical read head one or two spirals in front of the track containing the desired data and follow the spiral until the desired data is reached. Such a "reseek" results in a break or gap in the data transferred from the CD drive to the multimedia computer, and thus a break or gap in the audio and video signals processed by the multimedia computer.
To compensate for problems associated with reseeks on a CD drive, and to reduce or eliminate the breaks or gaps in data being read and transferred by a CD drive, known optical drives include a cache random access memory (RAM) associated with the drive's internal processor to temporarily store data that is read from a CD. When the drive's processor requests particular data, it is read and stored in the cache RAM until an external host computer, such as a multimedia computer, requests the particular data. When the host computer requests the particular data, it is accessed directly from the cache RAM in the drive. Depending on the size of the cache RAM, and thus the amount of data that may be stored therein, the number of reseeks required is substantially reduced or eliminated. This improves the reader's throughput to the host computer and eliminates the breaks or gaps in presenting data to the host computer caused by delays associated with locating a particular data block or caused by reseeking of data.
There are numerous disadvantages in providing an optical drive with cache RAM. Cache RAM is very expensive and adds to the cost of the drive. Depending on the amount of cache RAM required, the cost of the cache RAM could equal that of the drive. In addition, the amount of cache RAM necessary to limit reseeks differs with the particular application, making it difficult to provide a single drive product adaptable for universal use. Further, a drive adaptable to manage a variety of different cache RAM sizes would result in a more complex arrangement.